1. Field of the Invention
The invention relates to a method and a device for controlling a battery pulse heating mode of a traction battery of a hybrid vehicle.
2. Description of the Related Art
The traction battery (for example NiMH or Li-ion technology) of a hybrid vehicle often cannot provide sufficient electrical current for the hybrid functions at low temperatures. More particularly, the maximum admissible electrical current limits for charging and discharging are a function of the temperature of the traction battery. Thus, the hybrid functions are restricted at low accumulator temperatures or electric driving is available only beyond a certain temperature threshold, for example above 10° C. Accordingly, the fuel saving capability of the hybrid system is reduced at low temperatures.
To solve this problem, the traction battery is heated to its optimum operating temperature as quickly as possible to ensure the optimum availability of the hybrid functions. An air-cooled traction battery can be heated only by its own losses during charging and discharging. Heating-up during driving necessitates a battery pulse heating mode where charging and discharging occur within the temperature-dependent electrical current limits.
The battery pulse heating mode is implemented with the internal combustion engine being assisted by the electric machine during driving for discharging, and being acted on by the electric machine with a load point increase for charging. However, these purposely generated power losses for heating the traction battery require additional fuel so that over short driving distances the average fuel consumption is considerably higher with the battery pulse heating mode than without.
FIG. 5 shows dependencies of the additional fuel consumption ΔV during a battery pulse heating mode on the distance W travelled for different traction battery temperatures T=−10, −5, 0, +5° C.
In FIG. 5, curve A denotes a traction battery temperature T=−10° C., curve B denotes a traction battery temperature T=−5° C., curve C denotes a traction battery temperature T=0° C., and curve D denotes a traction battery temperature T=5° C. As can be seen from FIG. 5, the additional consumption is significantly higher at a relatively low battery temperature than at a relatively high traction battery temperature. In other words, at relatively low traction battery temperatures, a greater distance must be travelled to arrive at a correspondingly reduced level of additional consumption.
For simplicity, it is assumed in the illustration of FIG. 5 that the battery pulsing takes place at infinite speed, which is of course not the case in reality, but can be assumed here for simplification of the illustration.
U.S. Pat. No. 6,271,648 B1 discloses a method for conditioning a battery to improve the cold-start behavior of a vehicle. The method uses the electric starter to carry out a battery pulse heating mode at cold temperatures, before the vehicle is started.
JP 2007097359 A discloses a device for controlling a battery pulse heating mode, wherein a navigation destination is taken into consideration for the decision as to whether the battery pulse heating mode is carried out.
JP 2008162318 A discloses a method for controlling a battery pulse heating mode on the basis of a route identification.
It is an object of the present invention to provide a more flexible method for controlling a battery pulse heating mode of a traction battery of a hybrid vehicle, and a corresponding device.